src/test/ui/nll/relate_tys/var-appears-twice.rs

   1 // Test that the NLL `relate_tys` code correctly deduces that a
   2 // function returning always its first argument can be upcast to one
   3 // that returns either first or second argument.
   4
   5 use std::cell::Cell;
   6
   7 type DoubleCell<A> = Cell<(A, A)>;
   8 type DoublePair<A> = (A, A);
   9
  10 fn make_cell<'b>(x: &'b u32) -> Cell<(&'static u32, &'b u32)> {
  11     panic!()
  12 }
  13
  14 fn main() {
  15     let a: &'static u32 = &22;
  16     let b = 44;
  17
  18     // Here we get an error because `DoubleCell<_>` requires the same type
  19     // on both parts of the `Cell`, and we can't have that.
  20     let x: DoubleCell<_> = make_cell(&b); //~ ERROR
  21
  22     // Here we do not get an error because `DoublePair<_>` permits
  23     // variance on the lifetimes involved.
  24     let y: DoublePair<_> = make_cell(&b).get();
  25 }